1. Field of the Invention
The present invention generally relates to the field of thermophotovoltaic (TPV) direct energy conversion and more particularly, to an improved TPV semiconductor device that increases conversion efficiency and that incorporates a heavily doped n-type region which provides partial below bandgap spectral control.
2. Related Art
Thermophotovoltaic (TPV) energy systems convert thermal energy to electric power using the same principle of operation as solar cells. In particular, a heat source radiatively emits photons which are incident on the TPV device. The TPV device includes a semiconductor p-n junction, wherein the top layer is referred to as the emitter and the bottom layer is referred to as the base. Photons with energy greater than the semiconductor bandgap, E.sub.g (typically ranging from 0.50 eV to 0.73 eV for TPV devices), excite electrons from the valence band to the conduction band (interband transition). The resultant electron-hole pairs are then collected by metal contacts and can be used to power electrical loads. Photons with an energy less than the semiconductor bandgap are parasitically absorbed as heat. In order to increase the efficiency of a TPV system, some form of spectral control is used to reflect photons, with energy less than the semiconductor bandgap, back to the radiator, where they are recycled.
In U.S. Pat. No. 5,403,405, to Fraas et al., TPV devices are disclosed which include relatively inefficient short pass IR filters for reflecting photons having an energy level below that of the semiconductor bandgap. In one embodiment, disclosed in the Fraas et al. patent, a dielectric filter is provided which has multiple layers which transmit some of the below bandgap radiator energy, which is absorbed as heat in the TPV device and reduces the efficiency of the TPV devices. In another embodiment, a plasma filter is used which has a plasma wavelength .lambda..sub.p, ranging between .lambda..sub.g and 2.lambda..sub.g, where .lambda..sub.g is the characteristic wavelength of the TPV semiconductor layers. Plasma filters having a plasma wavelength .lambda..sub.p between .lambda..sub.g and 2.lambda..sub.g exhibit a significant amount of absorption of photons which have an energy level greater than the semiconductor bandgap.